Removing the harmful pollutants in engine exhaust has been an intense focus of the automotive, trucking, and off-road engine industries over the last several decades. Increasingly stringent emissions regulations worldwide have driven the introduction of engine improvements and catalysts and other aftertreatment for emissions control. In particular, the emissions regulations for fuel-efficient diesel engines that were implemented in 2007 and 2010 have resulted in a new generation of emissions control products. Among the products are catalysts that oxidize pollutants such as CO and hydrocarbons (HCs); these catalysts usually reach 90% conversion of pollutants between 200° C. and 350° C., but below these temperatures, the catalysts are relatively inactive.
Consequently, more than 50% of a vehicle's pollutant emissions occur in the first 2-3 minutes of the transient drive cycle required for certification and under “cold-start” or idling conditions. Thus, as emissions regulations become more stringent, meeting the emission regulations will require increased activity during this warm-up period.
To further complicate matters, the increased Corporate Average Fuel Economy standards that will be implemented over the next decade will result in the introduction of more fuel-efficient engines. Higher fuel efficiency will result in less heat lost to exhaust and lower exhaust temperatures, which further necessitates the need for increased emissions control activity at low temperatures. With this in mind the U.S. DRIVE Advanced Combustion & Emissions Control Technology Team has set a goal of achieving 90% conversion of CO/HC/NOx at 150° C. Such an aggressive goal is designed to address the challenges associated with meeting U.S. Environmental Protection Agency Tier 3 emission regulations for light-duty vehicles which phases in between 2017 and 2025 as well as other new emission regulations across the world.
Although great progress has been made through decades of research and development on the existing material combinations used for oxidation catalysts, further increase of low temperature performance is difficult. The platinum group metals (PGMs) Pt, Pd, and Rh are the active component in essentially all commercial oxidation catalysts. Increasing PGM loadings may help to increase the catalytic efficiency, but as PGM content is increased, maintaining a highly dispersed PGM surface becomes more difficult. This is due to the problem that as more PGM is added, larger PGM particles result which have less surface area to mass than the more finely dispersed smaller PGM particles associated with lower PGM loadings on catalysts. Since all catalytic reactions occur on the surface, the decreased surface area per added PGM causes the approach of increasing PGM content to be too expensive for long term success.
Furthermore, while PGM materials are active for both CO and HC oxidation reactions, both CO and HC readily chemisorb to the PGM surface which can create competition between the species for access to PGM sites where the oxidation process occurs. Such competition that decreases catalytic activity is known as “inhibition” and has been a major limiting factor in the low temperature activity of catalysts for engine emission control. Essentially, HCs in the exhaust stream can bind to the PGM surface and, at temperatures where no reactivity of HCs occurs, fully occupy the surface thereby preventing adsorption and reaction of CO. The same inhibition process can occur in reverse with CO masking PGM access to HCs. Thus, oxidation of CO and HCs is much more difficult when both pollutants are in the exhaust stream especially at low temperatures where oxidation reactions of either species does not occur rapidly.
It is important to note that a new family of advanced combustion variants for engines are being developed that attain higher fuel efficiency via increasing the homogeneity of combustion which lowers the internal combustion process temperatures in the engine cylinder. Such combustion techniques are known as “low temperature combustion” and have the benefits of higher fuel efficiency (as compared with conventional diesel combustion) and lower NOx and particulate matter emissions (due to the more homogenous combustion process and lower combustion temperatures). However, as expected, the exhaust observed from low temperature combustion engines has been shown to have lower temperature (as compared with conventional diesel engine exhaust) and increased CO and HC emissions (as more fuel components escape combustion due to the homogeneous charge and lower combustion temperatures). For these promising combustion techniques, the combination of lower exhaust temperatures and the potential for inhibition between CO and HC oxidation over PGM catalysts creates an even greater challenge.
In summary, the utility of existing PGM-based oxidation catalysts is insufficient for the combination of new emission requirements and changing exhaust conditions of new fuel-efficient engine technologies.